Please relate growing cells to these workers. If there are excess cells, all of the parts are processed. If these were cells, however, there would not be enough food resources to sustain them all, and their numbers would drop. On the other hand, with a good balance of workers and parts, an increase in the rate of parts would over employ them, and some parts would be wasted. If these were cells, there would be a delay while growth made more workers.
The very important point is the difference in response to a step up and to a step down in rate of supplying parts. Decrease supply rate (concentration of nutrient) to get an immediate decrease in processing rate. Increase supply rate to get maybe a slight increase in processing rate, but it takes time to get the full response as more workers are created.